Incomplete combustion of heavy hydrocarbon compounds, such as heavy oils, Diesel oil, and the like leads to soot formation. In the operation of Diesel engines, which are increasingly used in automotive application, "smoking" of the engine leads to ambient air pollution although the carbon monoxide and hydrocarbon emission of the exhaust gases from the Diesel engine itself are very low. Thus, in spite of the high operating efficiency and low fuel consumption of Diesel engines, their use in automotive application has been held back due to emission of visible black exhaust from improperly operating or adjusted engines. It is, therefore, desirable to detect the formation of soot or unburned carbon particles in the exhaust of the engine and to provide an output signal which then can be used to indicate the presence of soot and further to be used as a sensing or control signal, for example in a control loop, to automatically adjust air, or the supply of fuel for complete combustion and to prevent excess carbon formation. It is possible to sense the transparency of output gases optically; such installations, however, are more suited to fixed locations, for example for smokestacks, and are not readily adaptable to automotive use, particularly in view of the rugged and highly variable ambient operating conditions to which they are objected, including wide swings in temperature, temperature gradients, shock, vibration, and the like. It would, therefore, be desirable to provide a sensor which changes a characteristic directly upon the formation of soot which can be deposited on a sensing element. In the past, soot deposits on a sensing plate have been used, which changes the electrical conductivity of a sensor by the formation of a carbon bridge between electrodes. Such sensors are not suitable for continuous indication of carbon content since sensing is not reversible, that is, after formation of a carbon bridge, the sensor cannot immediately or with suitable response speed sense the absence of further carbon formation since the bridge of soot formed between the electrode will remain and cannot be promptly and effectively removed.